1. Field of the Invention
The invention relates generally to piezoelectric transducers and more particularly to ceramic granule-polymer composites for large area transduction and to methods of making such composite transducers.
2. Background of the Related Art
Piezoelectric transducers are devices that utilize a piezoelectric ceramic to convert mechanical energy into electrical energy or electrical energy into mechanical energy. Transducers have numerous practical applications including in hydrophones for detecting underwater acoustic waves.
An ongoing problem in developing hydrophones is finding piezoelectric material in which the desirable properties for underwater transducer applications are optimized. Ideally, the piezoelectric material should exhibit good hydrostatic response, should have a low weight and density, should be homogeneous and should be easy to manufacture in a large area configuration. Single-phase piezoelectrics, such as those made of single-phase lead zirconate titanate (PZT), a widely used piezoelectric material, are not well suited for use as monolytic large area hydrophones because of their weight, density, inflexibility and expense. Consequently, research on materials and structures for large area hydrophones has focused on developing composite configurations such as ceramic-polymer composites. In ceramic-polymer composites, a piezoelectrically active phase, typically a piezoelectric ceramic, is combined with a flexible, piezoelectrically inactive phase, typically an elastic polymer, that has desirable mechanical properties such as strength, low density and flexibility. The composite material is typically formed as a substantially two-dimensional sheet that is provided with relatively stiff top and bottom activation ("cover") plates.
In a two-phase system such as in a piezoelectric ceramic-polymer composite, the piezoelectric properties depend on the connectivity of the two phases. Connectivity of the phases of a composite is defined by the number of dimensions in which each phase is self-connected. Under a notation system described by Newnham et al, "Connectivity and Piezoelectric Composites", Mat. Res. Bull. Vol 13 (1978) pp 525-536, ten different types of connectivity are possible in a two-phase ceramic-polymer system: 0-0, 1-0, 2-0, 3-0, 1-1, 2-1, 3-1, 2-2, 3-2, and 3-3, with the first numeral referring to the self-connectedness of the ceramic and the second numeral referring to the self-connectedness of the polymer.
The simplest type of piezoelectric ceramic-polymer composite is a composite with 0-3 connectivity, that is, a composite that has discrete non-connected piezoelectric ceramic particles surrounded by a polymer matrix that is self-connected in three dimensions. Ceramic-polymer composites having 0-3 connectivity are described in U.S. Pat. No. 4,977,547 to Giniewicz et al, in W. B. Harrison et al, "Pyroelectric Properties of Flexible PZT Composites" Ferroelectrics, 1980, Vol. 27, pp 125-128, and in W. B. Harrison, "Flexible Piezoelectric Organic Composites", Proceedings of the Workshop on Sonar Transducer Material, Naval Research Laboratories P. 257, November 1975, all of the above incorporated herein by reference. Piezoelectric ceramic-polymer composites with 0-3 connectivity are relatively easy and inexpensive to make by the steps of mixing the ceramic particles with the polymer and then shaping and curing the mixture to form the composite. The disadvantage of 0-3 piezoelectric ceramics is that since the particles are generally disconnected from one another, the electrical and force field is always in part across piezoelectrically inactive material, and the overall piezoelectric activity is therefore less than that of the ceramic itself.
The overall piezoelectric response is improved in composites having 1-3 connectivity, that is, composites having a piezoelectric ceramic that is self-connected in one dimension (i.e., in the direction that force is applied), surrounded by a polymer phase that is self-connected in three dimensions. Typically, the ceramic phase is in the form of an array of ceramic rods aligned in the poling direction and held together by the polymer matrix. Cover plates may be disposed on opposing sides of the matrix transversely to the rods. Ceramic-polymer composites hating 1-3 connectivity are described in U.S. Pat. No. 5,527,480 to Bailey et al, U.S. Pat. No. 4,412,148 to Klicker et al, and U.S. Pat. No. 5,340,510 to Bowen, all of the above incorporated herein by reference. A disadvantage of 1-3 composites is that they are more complex structures that can be difficult and labor intensive to fabricate on a large scale. A simpler method of fabricating composites with 1-3 connectivity by covering piezoelectric ceramic spheres with a polymer and then sanding the polymer to expose the ceramic spheres is described in Safari et al, "Flexible Composite Transducers", J. Am. Ceram. Soc., 65: 207-209, 1982.
The 1-3 composites described above typically have the disadvantage of an inefficient transfer of force from the cover plates to the ceramic rods, because some of the force is applied to the coplanar polymer matrix instead of to the ceramic rods. U.S. Pat. No. 5,376,859, incorporated herein by reference, describes a transducer having rods that extend beyond the matrix and cover plates that are disposed so that they touch only the rods and do not transmit force to the polymer matrix. However, the transducer described is a complex structure that is difficult and labor intensive to manufacture.